Numerous vehicles of the track or wheel type to perform various material handling operations have been proposed. One type of vehicle that recently has received a remarkable degree of attention is a small unit that incorporates four wheels which are driven by two separate power sources and the steering or turning movement is accomplished by driving the pair of wheels on one side of the vehicle in one direction while the second pair of wheels are either in a neutral condition or driven at a reduced speed or in the opposite direction. These vehicles have generally been referred to as skid steer vehicles.
One such type of vehicle that has been commercially accepted includes a hydraulic drive system for driving the respective wheels. Usually the drive system includes a pair of pumps that are driven by the engine or propulsion unit and a pair of hydraulic motors that are respectively operatively connected to the two pairs of wheels on opposite sides of the vehicle.
In order to control the speed and the direction of movement of the vehicle, it has been customary to utilize a variable displacement reversible pump cooperating with a fixed displacement motor for each pair of wheels and the output of the pumps is controlled by separate manual control mechanisms, such as control levers. Such type of system is disclosed in U.S. Pat. No. 4,030,560 which is assigned to the assignee of the present invention.
Another type of hydraulic circuit for driving vehicles of this type is disclosed in U.S. Pat. No. 3,870,172 wherein fixed displacement dual gear pumps are driven by an engine and respectively have a constant output which is supplied to left and right-hand motors and the control includes valves incorporated in the circuit between the pumps and the motors which will again control the speed and direction of the vehicle by controlling the speed and direction of the output shafts of the variable speed motors.
Presently, the assignee of the present invention is commercially manufacturing various sizes of machines all of which include a tandem pump assembly including two variable displacement pumps and two fixed displacement piston motors with the variable displacement pumps being respectively controlled by manual control levers in the operator's station. Thus, by movement of the respective control levers for driving the ground engaging members on opposite sides of the vehicle, the speed and direction of the vehicle can be controlled. While such a unit has been accepted in the marketplace, manufacturers are constantly striving to increase the versatility of such vehicles and also maintain or reduce the cost thereof.
In this respect, it has recently been proposed to convert a wheeled type vehicle of the type shown in the above-mentioned patents to a crawler type vehicle to increase the flotation and tractive efforts for the vehicle in certain operating conditions.
One other area that has received a substantial amount of attention is the versatility of the speed of the vehicles. Because of the compact nature of the machine and the competitive selling price thereof, manufacturers are to some extent limited by the size and complexity of the pumps which can be incorporated into the unit to drive the vehicle. This factor has to some extent limited the versatility of the vehicle because competitively priced pumps have limitations respecting the variations in outputs of the unit. This in turn limits the maximum speed of the vehicle to something less than four miles per hour. This factor significantly impedes the efficiency of a machine of this type, particularly when it is being utilized for hauling loads a substantial distance.